1. Field of the Invention
The present invention relates to a radiation image pickup apparatus having a two-dimensional sensor which is provided with a conversion element for converting radiation into electric charge and a transferring unit for transferring an electric signal according to the electric charge to the outside and a plurality of pixel are two-dimensionally arranged and its control method and a radiation image pickup system.
2. Description of the Related Art
Because of advancement of semiconductor technology, radiation image pickup apparatuses respectively using a thin-film semiconductor element are practically used and generalized in recent years.
In the case of these radiation image pickup apparatuses, it is possible to perform not only still image radiographing but also moving image radiographing such as fluoroscopy or CT radiographing which can be conventionally performed by only an image intensifier (I·I), CCD or image pickup apparatus using a special solid-state image sensing device by increasing the speed of the reading operation or raising the sensitivity.
When moving image radiographing can be made by the above-described radiation image pickup apparatus, still image and moving image can be radiographed by one apparatus. Therefore, also when performing still image radiographing while radiographing a moving image, it is possible to efficiently perform the still image radiographing. Moreover, it is possible to perform various radiographings such as fluoroscopy and tomography by one apparatus and a medical care site becomes efficient. Therefore, a radiation image pickup apparatus capable of efficiently radiographing still image and moving image at high quality is requested.
However, to realize a radiation image pickup apparatus purposed from still image radiographing up to moving image radiographing such as fluoroscopy and CT radiographing, antinomic characteristic between resolution requested for still image radiographing and sensitivity requested for moving image radiographing must be realized.
In the case of a radiation image pickup apparatus, the resolution depends on the composition and thickness of phosphor and layer configuration between phosphor and photoelectric conversion element and pixel pitch. Particularly, the pixel pitch must be approx. 100 to 200 μm to realize a resolution requested for still image radiographing.
In the case of moving image radiographing, it is desired that a signal/noise ratio (hereafter referred to as S/N) and a frame rate are high. As the reason, in the case of moving image radiographing, X rays must be applied to a patient for a long time. Therefore, the application dosage of radiation for unit time is restricted to approx. 1/10 to 1/100 of still image radiographing. Therefore, to realize S/N necessary for diagnosis in moving image radiographing, it is requested that the sensitivity of a conversion device is high. However, for resolution, it is not necessary that the sensitivity is fine as still image radiographing. Particularly in the case of CT radiographing, an image is operated by a computer and converted into a tomographic image of a human body. Therefore, the resolution every 100 to 200 μm is not necessary. Moreover, in the case of fluoroscopy, necessary resolution depends on a unit to be radiographed or radiographing purpose and resolution is not greatly requested for preview radiographing for still image radiographing.
In general, to improve the S/N of a conversion device, it is a simple method to increase a pixel in size (increase pixel pitch) and increase the sensitivity which is a signal value per pixel. However, increase of a pixel in size results in deterioration of resolution and the resolution requested for still image radiographing cannot be satisfied.
Therefore, in the case of a radiation image pickup apparatus having pixels of a pixel pitch optimized for still image radiographing, when performing moving image radiographing, a plurality of pixels are simultaneously read to perform pixel addition for adding pixel outputs. Thereby, it is possible to apparently handle the pixels as a large pixel. By using this method, it is possible to solve the problem of the resolution requested for still image radiographing and S/N requested for moving image radiographing which are an antinomic problem. In this case, the S/N improved by pixel addition is approx. vn times (n is the added number of pixels) larger than the S/N when pixel addition is not performed.
This pixel addition is disclosed in, for example, Japanese Patent Application Laid-Open No. H07-322141.